Mess up Definition 1 of 4Pro Tip 1/2Common Confusion Don't confuse 'mess up' with 'clean up'. 'Mess up' often unintentionally leads to mistakes. I meant to help, but I only messed up the plan more. Pro Tip 2/2Informal Usage 'Mess up' is casual and mostly used in informal speech or writing. Sorry I'm late, I messed up the bus schedule.